Method and apparatus for separating oil from water in wastewater containing an emulsified oil

ABSTRACT

Wastewater containing a surfactant and an oil content that has been emulsified by the action of the surfactant can be freed of the oil content by a method including feeding the wastewater into the anode compartment, for electrolysis, of a diaphragm electrolyzer having an anode and a cathode provided in the anode compartment and a cathode compartment, respectively, which are spaced apart by a porous diaphragm and which are supplied with a dc voltage between the anode and the cathode, passing part of the electrolyzed wastewater through the diaphragm so that it enters the cathode compartment, discharging the influent from the cathode compartment, discharging the remainder of the electrolyzed wastewater from the anode compartment and introducing the same into the intermediate portion of a gas-liquid separator, withdrawing part of the influent from the top of the gas-liquid separator and introducing the same into a layer packed with an adhering material, where it is brought into contact with the adhering material, directing the effluent to an oil-water separation step for accomplishing the intended oil-water separation, and withdrawing the remainder from the bottom of the gas-liquid separator and returning the same to mix with the feed to the electrolysis step. Preferably, the polarities of the two electrodes are changed alternately at specified time intervals during the step of diaphragm electrolysis such that the anode compartment is switched to the cathode compartment and vice versa.

BACKGROUND OF THE INVENTION

This invention relates to the separation of oil from water in wastewatercontaining an emulsified oil. More particularly, the invention relatesto a method and an apparatus for performing oil-water separation bydiaphragm electrolysis of water-base cleaning solutions, liquid wastewater-soluble cutting oils and coolants that contain surfactants andoils at the same time.

As a result of the recent revision of the London Treaty on the oceandisposal of wastes, it is no longer possible to dump liquid wastewater-soluble cutting oils and coolants into the sea which has been themethod of disposal so far. Hence, it is urgently required to develop atechnology that provides for economical land disposal of these kinds ofwastewater.

Chlorine-base organic solvents such as Flon and trichloroethane whichhave hitherto been extensively used as industrial detergents are nowrecognized as ozone layer depleting substances and it was agreedinternationally to prohibit the production of these substances by theend of 1995. Under these circumstances, the development of alternativedetergents to Flon and trichloroethane is a peremptory need and they arebeing replaced by water-base detergents containing surfactants andalkalies as main components, quasi-water-base detergents comprising amixture of water with organic solvents such as alcohols and glycolethers, and non-water-base detergents typified by hydrocarbon-basesolvents.

However, these substitute detergents have their own problems. To beginwith, non-water-base detergents suffer from the disadvantage of highrunning costs since the cleaning operation is performed with thesedetergents alone. In addition, most of these non-water base detergentsare inflammable, so that the cleaning apparatus must be designed to beexplosion-proof (which increases its price) or cannot be of large size.

The water-base and quasi-water-base detergents, particularly, thewater-base detergents, feature low running costs since they are dilutedwith large volumes of water before use. In addition, they are notpotentially a hazard, so a large cleaning apparatus can be constructedeasily at a fairly low cost. On the other hand, the use of large volumesof water requires that a water treatment unit capable of oil-waterseparation of cleaning solutions and ecologically acceptable dischargeof rinse water should be installed as an essential component of theoverall cleaning system. Consider, for example, the case of cleaningworkpieces with water-base detergents. As the cleaning operationproceeds, contaminants such as oils from the workpiece build upgradually in the cleaning solution to reduce its detergency. Naturally,in order to extend the service life of the cleaning solution whilemaintaining its detergency, contaminants such as oils must be constantlyremoved from the cleaning solution.

Conventional methods of performing oil-water separation on water-basedetergents include: an emulsion breaking and floating separation methodwhich employs a chemical such as an emulsion breaker; an electrostaticseparation method; a coalescer method which accelerates the coalescingand coarsening of oil particles; and a membrane separation method whichemploys an ultrafiltration membrane or a microfiltration membrane.

However, these conventional techniques have their own problems. In theemulsion breaking and floating separation method which employs anemulsion breaker, the cleaning solution which has been subjected to theoil-water separation treatment has no detergency and is not suitable forsubsequent use. In the electrostatic separation method and the coalescermethod, the intended effect of oil-water separation is not attained ifthe oil content of the cleaning solution is present as fine emulsionparticles. The use of an ultrafiltration membrane or a microfiltrationmembrane has the disadvantage of removing not only the oil but also thedetergent component and, in addition, an expensive apparatus has to beemployed.

Water-base detergents are typically composed of surfactants as a maincomponent which is responsible for detergency, as well as rustinhibitors, antifoaming agents, and organic or inorganic builders suchas alkali components. The surfactants which are responsible fordetergency may be nonionic or anionic but from a detergency viewpoint,nonionic surfactants having cloud points in the range from 30° to 60° C.are often used. Prior to use, the water-base detergents are diluted withwater to a specified concentration, thereby formulating aqueous cleaningsolutions.

At temperatures below their cloud points, nonionic surfactants dissolvein water and exhibit surface activity; however, at temperatures abovetheir cloud points, the hydrophilic groups are dehydrated and themolecules associate with themselves to cause the loss of surfaceactivity. At even higher temperatures, the nonionic surfactantsprecipitate either as a floc or in a liquid form. Conversely, if thetemperature drops below the cloud point, the hydrophilic groups in thesurfactant which have been insoluble are hydrated to become watersoluble again, thereby restoring the surface activity.

Therefore, if an aqueous cleaning solution containing a nonionicsurfactant as a main component is heated to a temperature higher thanthe cloud point, the surfactant will lose its surface activity and theoil content will float to separate from the water. However, if thecleaning solution has an oil contaminant, the surfactant will notprecipitate on account of the interaction with the oil; on the contrary,it floats together with the oil content, making it impossible todischarge only the oil content from the system, which is the inherentobject of the oil-water separation treatment.

There has been previously proposed a floating separation technology thatcould accomplish efficient oil-water separation of an aqueous cleaningsolution even in the case where the latter was contaminated with an oil.

According to such technology, both a nonionic surfactant having a cloudpoint of 40°-70° C. and a nonionic surfactant having a cloud point of20°-40° C. were incorporated in a water-base detergent and thecontaminant such as oil in the cleaning solution could be separated bymerely heating it to a temperature above the cloud point of thedetergent. The technology also included a method for oil-waterseparation of the aqueous cleaning solution that has been used in thecleaning operation.

The technology works very effectively for cleaning solutions thatcontain water-insoluble oils and those which contain water-insoluble andnon-emulsifiable oils; however, it has not been applicable to cleaningsolutions that contain water-soluble oils or those which containemulsifiable water-insoluble oils that incorporate anionic surfactants.

Oils used in the machining of metal parts consist of water-soluble oilstypified by water-soluble cutting oils and coolants, and water-insolubleoils typified by press working oils and rolling mill oils. Thewater-insoluble oils are classified as an emulsifiable type and anon-emulsifiable type. The water-soluble oils typified by water-solublecutting oils and coolants have high contents of anionic surface activesubstances such as sodium alkylsulfonates and some of thewater-insoluble oils contain large amounts of calcium sulfonate andother anionic surface active substances as rust inhibitors. Thewater-base detergent previously developed and the method of oil-waterseparation based on the heating of such water-base detergent have beeninapplicable to those oils which contain large amounts of anionicsurface active substances.

SUMMARY OF THE INVENTION

In order to deal with these problems, the present inventors conductedintensive studies and accomplished an improved method for oil-waterseparation of cleaning solutions. The present inventors made furtherimprovements of this basic technology (the first aspect of theinvention), which will be described below as the second to the fourthaspects in the order of development.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an example of the apparatus for performingoil-water separation according to the first aspect of the invention;

FIG. 2 shows schematically an example of the apparatus for performingoil-water separation according to the second aspect of the invention;

FIG. 3 shows schematically an example of the apparatus for performingoil-water separation according to the third aspect of the invention;

FIG. 4a is a graph showing the profiles of oil content obtained inExample 5 and Comparative Example 5;

FIG. 4b is a graph showing the profiles of TOC obtained in Example 5 andComparative Example 5;

FIG. 4c is a graph showing the profiles of electrolytic voltage obtainedin Example 5 and Comparative Example 5;

FIG. 5 shows schematically an example of the apparatus for performingoil-water separation according to the fourth aspect of the invention andused in Example 6;

FIG. 6 is a graph showing the profiles of oil content obtained inExample 6 and Comparative Example 6.

DETAILED DESCRIPTION OF THE INVENTION

(1) First Aspect

The first and the core aspect of the invention relates to a method foroil-water separation of a cleaning solution in an oil-water separationstep as it comes from a work cleaning step together with an oilycontaminant, which comprises the steps of feeding the contaminatedcleaning solution into the anode compartment of a diaphragm electrolyzerthat is supplied with a dc current due to the application of a dcvoltage between an anode and a cathode, heating the cleaning solutionafter it has passed through the anode compartment, introducing theheated cleaning solution to the oil-water separation step, where oil isseparated from the water, and feeding the oil-free cleaning solutioninto the cathode compartment of the diaphragm electrolyzer.

The following are preferred embodiments of the first aspect of theinvention.

i) The cleaning solution is water.

ii) The cleaning solution is water having a surfactant incorporatedtherein.

iii) The cleaning solution is water having a nonionic surfactantincorporated therein.

iv) The cleaning solution is water having incorporated therein both anonionic surfactant having a cloud point of 20°-40° C. and a nonionicsurfactant having a cloud point of 40°-80° C.

v) The cleaning solution is water having incorporated therein a nonionicsurfactant having a cloud point of 20°-40° C., a nonionic surfactanthaving a cloud point of 40°-80° C., and a builder.

vi) The builder is sodium sulfate.

The basic method of oil-water separation and the preferred embodimentsdescribed above can be effectively implemented by an apparatus whichcomprises a cleaning solution tank, a diaphragm electrolyzer having ananode and a cathode provided in an anode compartment and a cathodecompartment, respectively, that are spaced apart by a porous diaphragm,and an oil-water separation tank and which is so adapted that acontaminated cleaning solution is fed from the cleaning solution tankinto the anode compartment of the diaphragm electrolyzer, from which itis fed to the oil-water separation tank through an inlet and emergestherefrom through an outlet to be fed into the cathode compartment ofthe diaphragm electrolyzer.

The cleaning solution to be used in the above-described methodpreferably contains builders that are not only effective for rustinhibiting and antifoaming purposes but also are capable of enhancingelectrical conductivity. Preferred builders are those which will notexperience any chemical changes during electrolysis, as exemplified bysodium sulfate and potassium sulfate.

In the first aspect of the invention, the cleaning solution as it passesthrough the anode compartment of the diaphragm electrolyzer is oxidizedwith nascent oxygen and otherwise sterilized, so it can be put toservice for a prolonged period of time without putrefaction even if nosterilizing agents are added.

The workpiece to which water-soluble cutting oils have adhered isoccasionally cleaned with either water alone or in the presence ofsodium sulfate. The oil-separation method according to the first aspectof the invention is also effective for this type of cleaning solution,as well as in the case where the cleaning solution is replenished duringthe cleaning operation.

In the preferred case ii), the cleaning solution is water having asurfactant incorporated therein and the surfactant is typically anionicor nonionic; if necessary, a mixture of anionic and nonionic surfactantsmay be employed. It should, however, be mentioned that the use ofanionic surfactants is preferably minimized since they have an acidbuffering action and make it difficult to achieve the intended pHadjustment of the cleaning solution if they are contained in largeamounts.

Preferred anionic surfactants include sodium alkylsulfonates and sodiumalkylcarboxylates. Preferred nonionic surfactants include various typessuch as polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether,polyethylene glycol, sorbitan fatty acid ester, polyoxyethylene sorbitanfatty acid ester and pluronic types.

Of the two types of nonionic surfactants that may be used in theinvention, the one having the lower cloud point (20°-40° C.) preferablyhas a cloud point in the range of 25°-30° C.

The first aspect of the invention has the following two majorcharacteristic features.

1) A diaphragm electrolyzer is provided after the step of cleaning theworkpiece and the cleaning solution from the cleaning step whichcontains an oily contaminant is fed into the anode compartment of thediaphragm electrolyzer, where it is rendered acidic, and the cleaningsolution is then fed into the oil-water separation step, where it isfreed of the oil contaminant, and subsequently it is fed into thecathode compartment of the diaphragm electrolyzer, where it is revertedto the initial pH.

2) Due to the presence of an anionic surfactant such as a sodiumalkylsufonate which is incorporated in a large amount in a water-solubleoil or of an anionic surface active substance such as calcium sulfonatewhich is incorporated in a water-insoluble oil, the oily contaminantwhich has been cleaned and dispersed by means of the nonionic surfactantwhich is the main component of the detergent used in the inventioncannot be separated from water if it is heated to a temperature abovethe cloud point of said nonionic surfactant; however, according to thefirst aspect of the invention, the cleaning solution is rendered acidicby being fed into the anode compartment of the diaphragm electrolyzerand, as a result, the oil dispersing effect of the anionic surfactant oranionic surface active substance is inactivated in such a way that theintended oil-water separation is accomplished.

The effectiveness of the inventive method for the oil-water separationof a cleaning solution by employing a diaphragm electrolyzer will now bedescribed specifically with reference to the case where the cleaningsolution contains a nonionic surfactant and a builder as two maincomponents.

If a metal part is degreased and cleaned with the above-described typeof cleaning solution, the oil removed from the metal part is emulsifiedwith the nonionic surfactant to become dispersed in the cleaningsolution. If the cleaning solution contaminated by the emulsified oilcontent is heated to a temperature above the cloud point of the nonionicsurfactant, the emulsion breaks, causing the oil to be separated byflotation. However, if the contaminated cleaning solution furthercontains an anionic surfactant or an anionic surface active substance,(i) the cloud point of the nonionic surfactant in the cleaning solutionwill increase and, in addition, (ii) the anionic surfactant or surfaceactive substance works effectively to continue the emulsification of theoil.

Needless to say, the intended mechanism of oil-water separation will notfunction properly if the cloud point of the nonionic surfactant in thecleaning solution becomes higher than the temperature setting forheating in the oil-water separation step or if an unduly increasedamount of the oil is emulsified by the anionic surfactant.

The anionic surfactant exhibits its surface activity when it isdissociated to become anionic and only in this state can the anionicsurfactant emulsify the oil content. However, as is well known, if thepH of the water in which the anionic surfactant dissolves or in which itis dispersed as micelles becomes lower than its pKa value, it will notbe dissociated but hydrogen will bind to anionic groups, thereby causingthe loss of surface activity and rendering the surfactant to becomeinsoluble in water. This behavior is also shown by the anionic surfaceactive substance. In order to lower the pH of the water in which theanionic surfactant dissolves or in which it is dispersed as micelles, anacid may be added but this method is not preferred due to the generationof by-products such as salts.

In the first aspect of the invention, the cleaning solution containingthe emulsified oil is fed into the anode compartment of a diaphragmelectrolyzer and the water is electrolyzed in accordance with thereaction scheme set forth below, whereby hydrogen ions are generated inthe medium in the anode compartment:

    H.sub.2 O→1/2.sub.2 +2H.sup.+ +2e.sup.-

The generated hydrogen ions make the cleaning solution acidic.

If the pH of the cleaning solution is made lower than the pKa value ofthe anionic surfactant such as a sodium alkylsulfonate which is presentin the cleaning solution by thusly generating hydrogen ion, the sodiumalkylsulfonate will undergo the following reaction:

    R--SO.sub.3.sup.- +H.sup.+ →R--SO.sub.3 H

whereby the hydrogen ion binds to the anionic group to cause the loss ofsurface activity of the anionic surfactant, thereby making it insolublein water. In this way, the deleterious effect of the anionic surfactantor surface active substance on the progress of oil-water separation canbe reduced satisfactorily in the first aspect of the invention. Thecleaning solution may be rendered acidic to a pH value of 3-7 and, forpractical purposes, satisfactory results can be attained if the pH iswithin the range of 5-7.

The pH adjusted cleaning solution emerging from the anode compartment ofthe diaphragm electrolyzer is then fed into the oil-water separationtank, where the temperature of the cleaning solution is raised to aspecified value higher than the cloud point of the nonionic surfactantcontained in the cleaning solution so that the oil is separatedtherefrom; the oil-separated cleaning solution is fed into the cathodecompartment of the diaphragm electrolyzer and the water is electrolyzedin accordance with the reaction scheme set forth below, whereby hydroxylions are generated in the medium in the cathode compartment:

    2H.sub.2 O+2e.sup.- →H.sub.2 +2OH.sup.-

The generated hydroxyl ion can neutralize the acidic cleaning solution.

The diaphragm electrolyzer to be used in the first aspect of theinvention has the anode compartment partitioned from the cathodecompartment by means of an ion-permeable diaphragm and an anode and acathode are provided in the anode and cathode compartments,respectively. An organic microfiltration (MF) membrane having an averagepore size of 0.1-3.0 μm is usually employed as the ion-permeablediaphragm. Any insoluble electrode may be employed without particularlimitations but an electrode comprising a titanium or other metalsubstrate that is plated with platinum is advantageously used as theanode and cathode; if desired, less costly ferrite or stainless stealelectrodes may be used as the cathode.

If the cleaning solution is a water containing nonionic surfactanthaving a cloud point of 20°-40° C., the relative proportion of the ionicsurfactant to be incorporated may be in the range of 0.2-1 compared withthe nonionic surfactant used as the detergent; for instance, thecleaning solution having a detergent loading of 5% may incorporate 1-5%of the nonionic surfactant having a cloud point of 20°-40° C.

An example of the apparatus for performing oil-water separationaccording to the first aspect of the invention is shown in FIG. 1. Metalparts that have been subjected to press working and which have anemulsifiable water-insoluble oil film adhering to their surface arecleaned in a cleaning tank 1 by means of an aqueous solution thatcontains a sodium alkylsulfonate and a nonionic surfactant of apolyoxyethylene alkyl phenyl ether type as detergents and which hassodium sulfate added as a builder. The cleaning solution containing anoily contaminant such as the emulsifiable water-insoluble oil issupplied into a diaphragm electrolyzer 2 at an anode compartment 4containing an anode 3 via a feed pipe 10 equipped with a valve 11 and afeed pump 12. The diaphragm electrolyzer 2 is divided into the anodecompartment 4 and a cathode compartment 6 by a diaphragm 8 and theyrespectively contain the anode 3 and a cathode 5, which are suppliedwith a predetermined dc current from a power source 7. In the anodecompartment 4, water is electrolyzed to generate hydrogen ions, whichwill cause of the loss of the surface activity of the anionic surfaceactive substance such as the sodium alkylsulfonate in theoil-contaminated cleaning solution which has been fed into said anodecompartment.

The cleaning solution thus rendered acidic emerges from the anodecompartment 4 and passes through a pipe 15 to be fed into an oil-waterseparation tank 9 via an inlet 13; in the oil-water separation tank 9,the cleaning solution is heated to a temperature higher than the cloudpoint of the nonionic surfactant of a polyoxyethylene alkyl phenyl ethertype.

The cleaning solution in the oil-water separation tank 9 needs only tobe heated to a temperature in the range of 50°-80° C., preferably60°-65° C., so that the oil content is selectively separated byflotation.

The cleaning solution is allowed to stay within the oil-water separationtank 9 for a detention time of no more than about 20 minutes and theseparated oil content is continuously discharged out of the system toflow into an oil tank 14 via a connection pipe 16.

The oil-freed cleaning solution is withdrawn from the bottom of theoil-water separation tank 9 via an outlet 17.

The cleaning solution from which the oil content and part of thesurfactant have been removed flows through a pipe 18 connected to theoutlet 17 such that it is fed into the cathode compartment 6 of thediaphragm electrolyzer 2 in which the cathode 5 is provided. In thecathode compartment 6, water is electrolyzed to generate hydroxyl ions,which will neutralize the cleaning solution fed into said cathodecompartment. The neutralized cleaning solution leaves the cathodecompartment 6 and passes through a pipe 19 to return to the cleaningtank 1. If desired, the cleaning solution which has been adjusted to theinitial pH in the cathode compartment 6 may be directly discharged outof the system without being returned to the cleaning tank 1.

In the first aspect of the invention, the cleaning solution to betreated is forced by a feed pump to circulate in a single pass through aclosed loop starting with the cleaning tank, followed in order by theanode compartment, oil-water separation tank and cathode compartment andending with the cleaning tank. In order to break the emulsified oilparticles in the cleaning solution, its pH must be made lower than thepKa value of the anionic surfactant and its temperature higher than thecloud point of the nonionic surfactant within the single pass. To thisend, the throughput, namely, the quantity of circulating flow has to belowered as the pH or alkalinity of the cleaning solution increases.However, the smaller quantity of circulating flow reduces the flow rateof the liquid on the surface of the diaphragm, thereby increasing thelikelihood of the contamination of the membrane and accelerating theincrease of the electrolytic voltage.

To solve these problems, it is important to ensure that the quantity ofcirculating flow to the electrolyzer diaphragm, particularly to theanode compartment, is always kept at a high level regardless of thenature of the cleaning solution to be treated.

(2) Second Aspect

The second aspect of the invention has been accomplished in order tomeet the above-described requirement. It relates to a method foroil-water separation of wastewater containing a surfactant and an oilcontent that has been emulsified by the action of said surfactant. As inthe first aspect, the diaphragm electrolyzer used in the second aspecthas an anode and a cathode provided in an anode compartment and acathode compartment, respectively, which are spaced apart by a porousdiaphragm; however, in the second aspect, the anode compartment of thediaphragm electrolyzer is supplied with the feed wastewater, part ofwhich is passed through the diaphragm to enter the cathode compartment,from which it is discharged while the remainder of the wastewater isdischarged from the anode compartment, thereby accomplishing theintended oil-water separation.

According to the second aspect of the invention, there is also providedan apparatus for water-oil separation of wastewater containing asurfactant and an oil content that has been emulsified by the action ofsaid surfactant, which comprises a diaphragm electrolyzer having ananode and a cathode provided in an anode compartment and a cathodecompartment, respectively, that are spaced apart by a porous diaphragm,and a tank for oil water separation of the effluent from the anodecompartment, said diaphragm electrolyzer being connected to saidoil-water separation tank in such a way that the latter is supplied withthe feed wastewater, which is withdrawn from the bottom of saidoil-separation tank to be fed into the anode compartment of saiddiaphragm electrolyzer, with part of the effluent from the anodecompartment being passed through the diaphragm to enter the cathodecompartment, from which it is discharged while the remainder of theeffluent is returned to the oil-water separation tank.

The tank for oil-water separation of the effluent from the anodecompartment is preferably equipped with an electric heater.Additionally, a wastewater receiving tank or receptacle may be providedabove said oil-water separation tank and the two tanks are connected insuch a way that the wastewater can be fed into the separation tank dueto the difference in liquid level. In this case, the bottom of theoil-water separation tank is connected to the receiving tank via a feedpump and a return valve. If desired, a separate oil-water separationtank may be provided and connected in such a way that it can be suppliedwith the effluent from the cathode compartment; in this case, the bottomof said separation tank may be connected to the wastewater receivingtank. Furthermore, an oil receptacle may be provided in such a way thatit is connected to the top of each separation tank.

The wastewater receiving tank may be used either as a tank for receivingthe wastewater containing both the surfactant used to clean theworkpiece and the emulsified oil or as a tank in which the workpiece isdirectly cleaned.

In the method of oil-water separation according to the second aspect ofthe invention, the pH of the wastewater in the anode compartment can berendered lower than the pKa (acid dissociation index) of the anionicsurfactant in water by diaphragm electrolysis and the electricalpermeation of water from the anode compartment to the cathodecompartment combines with the filter effect of the differential pressure(0-0.2 MPa) between the anode and cathode compartments to enrich the oilcontent not only in the anode compartment but also in the associatedoil-water separation tank. The factor of enrichment is adjusted to be inthe range of 0.5-5. Electrical permeation is a phenomenon that occurswhen cations such as Na⁺ electrophorese from the anode to the cathodecompartment during electrolysis and it is characterized by thepermeation of water from the anode to the cathode compartment throughthe diaphragm (which in the present invention is a microfiltrationmembrane having an average pore size of 0.1-3.0 μm).

If desired, sodium phosphate or sodium tripolyphosphate and/or sodiumsulfate may be incorporated in an amount of up to 30 mM in thewastewater to be treated and this is effective not only in preventingscale deposition on the cathode surface and the diaphragm on account ofthe hardness components such as calcium, magnesium and barium but alsofor increasing the electrical conductivity of the wastewater to betreated.

According to another feature of the second aspect, the effluent from theanode compartment is returned to the associated oil-water separationtank, whereby the reaction for insolubilizing the anionic surfactant byreducing the pH is carried out not in the anode compartment but in thetank for oil-water separation of the effluent from the anode compartmentand, in addition, the liquid flow rate on the side of the diaphragmfacing the anode compartment is increased to 0.1-0.5 m/sec. These areeffective not only in preventing the plugging of the diaphragm but alsofor maximizing the efficiency of oil separation in a single pass of theeffluent from the anode compartment to the oil-water separation tank,thereby enhancing the capacity for oil separation per unit electrolyticcurrent.

If an electric heater provided in the tank for oil-water separation ofthe effluent from the anode compartment is used to heat the effluent toa temperature above the cloud point of the nonionic surfactant, theefficiency of oil separation can be enhanced; in addition, the effluentfrom the cathode compartment may be introduced into the associatedoil-separation tank, whereby the oil contaminant passing through thediaphragm can be separated by flotation to achieve a further improvementin the efficiency of oil-water separation.

Surfactants that may be present in the wastewater to be treated inaccordance with the second aspect of the invention include anionicsurfactants such as organic sodium carboxylates, organic sodiumsulfonates and organic sodium sulfate, and nonionic surfactants ofvarious types such as polyoxyethylene alkyl phenyl ether,polyoxyethylene alkyl ether, polyethylene glycol, sorbitan fatty acidester, polyoxyethylene sorbitan fatty acid ester and pluronic types.Inorganic builders that may be present in the wastewater include variouskinds of sodium phosphate, sodium silicate and sodium borate.

The wastewater to be treated has preferably an alkalinity in the rangeof 0.1-10.0. The term "alkalinity" as used herein means the quantity inmilliliters of 0.1N HCl required to titrate 10 ml of the wastewater feedto an end pH point of 4.8.

In the second aspect of the invention, the wastewater containing theemulsified oil is fed into the anode compartment of a diaphragmelectrolyzer and the water is electrolyzed in accordance with thereaction scheme set forth below, whereby hydrogen ions are generated inthe medium in the anode compartment:

    H.sub.2 O→1/20.sub.2 +2H.sup.+ +2e.sup.31

The generated hydrogen ions make the cleaning solution acidic.

If the pH of the electrolyzed wastewater is made lower than the pKavalue of the anionic surfactant such as a sodium alkylsulfonate which ispresent in the wastewater by thus generating hydrogen ion, the sodiumalkylsulfonate will undergo the following reaction:

    R--SO.sub.3.sup.31 +H.sup.+ →R--SO.sub.3 H

whereby hydrogen ion binds to the anionic group to cause the loss ofsurface activity of the anionic surfactant, thereby making it insolublein water. In this way, the deleterious effect of the anionic surfactantor surface active substance on the progress of oil-water separation canbe reduced satisfactorily in the second aspect of the invention. Thewastewater may be rendered acidic to a pH value of 1-7 and, forpractical purposes, satisfactory results can be attained if the pH iswithin the range of 3-7.

In the second aspect of the invention, electrical permeation is anotherdriving force to enrich the oil content. Stated more specifically, ascations such as Na⁺ electrophorese from the anode to the cathodecompartment, water permeates the diaphragm (having an average pore sizeof no more than 3 μm) to move from the anode to the cathode compartment,whereby the oil content of the electrolyzed wastewater in the anodecompartment or the associated oil-water separation tank is sufficientlyenriched.

The pH adjusted wastewater emerging from the anode compartment of thediaphragm electrolyzer is then fed into the oil-water separation tank,where the temperature of the electrolyzed wastewater is raised to aspecified value higher than the cloud point of the nonionic surfactantcontained in the wastewater so that the oil is separated therefrom; theoil-separated wastewater is returned to the anode compartment of thediaphragm electrolyzer.

The diaphragm electrolyzer and the porous diaphragm which are to beemployed in the second aspect of the invention are identical to thoseused in the first aspect.

An example of the apparatus for performing oil-water separationaccording to the second aspect of the invention is shown in FIG. 2. Areceptacle 1 holds wastewater containing an emulsified oil content. Thewastewater is fed into an oil-water separation tank 2 via an inlet 5.The tank 2 is equipped with an electric heater 3 and an air vent valve4. The wastewater emerging from the tank 2 via an outlet 6 is forced bya feed pump 9 into a diaphragm electrolyzer 13 at an anode compartment15 containing an anode 14. The diaphragm electrolyzer 13 is divided intothe anode compartment 15 and a cathode compartment 17 by a diaphragm 18and they respectively contain the anode 14 and a cathode 16, which aresupplied with a predetermined dc current from a power source 19. In theanode compartment 15, water is electrolyzed to generate hydrogen ions,which will cause the loss of the surface activity of the anionic surfaceactive substance such as the sodium alkylsufonate in the wastewaterwhich has been fed into said anode compartment.

The wastewater thus rendered acidic emerges from the anode compartment15 of the diaphragm electrolyzer 13 to enter the oil-water separationtank 2 via an inlet 7; in the separation tank 2, the effluent is heatedwith the electric heater 3 to a temperature higher than the cloud pointof the nonionic surfactant.

The effluent fed into the oil-water separation tank 2 needs only to beheated to a temperature in the range of 50°-80° C., preferably 60°-70°C., so that the oil content is selectively separated by flotation.

The effluent is allowed to stay within the oil-water separation tank 2for a detention time of no more than about 20 minutes and the separatedoil content is continuously or intermittently discharged out of thesystem to flow into an oil reservoir 8 whereas the aqueous phase isrecovered from the tank 2 via the outlet 6 to return to the anodecompartment 15.

That part of the electrolyzed wastewater in the anode compartment 15which has passed through the diaphragm 18 to flow into the cathodecompartment 17 is fed into another oil-water separation tank 20 via aninlet 21 and, in that tank 20, the oil content that has passed throughthe diaphragm 18 is separated and sent to the oil reservoir 8 whereasthe aqueous phase emerges from the bottom of the tank 20 via an outlet22 to be returned to the receptacle 1. Shown by 23 is an air vent valve.

Thus, the wastewater to be treated is subjected to oil-water separationas it circulates between the oil-water separation tank 2 and the anodecompartment 15 of the diaphragm electrolyzer 13. The circulation loop isfitted with a pressure gage 10 and a pressure regulating valve 11. Avalve 12 for returning the effluent from the anode compartment isoperated to withdraw the aqueous phase into the receptacle 1 such thatthe pH of the effluent is maintained at a predetermined constant level.

(3) Third Aspect

In FIG. 2 showing the apparatus for implementing the method of oil-waterseparation in accordance with the second aspect, the electrolyzedwastewater from the anode compartment 15 is directly connected to thetank 2 for oil-water separation of the effluent from the anodecompartment and both the anode 14 and the cathode 16 are fixed inposition.

Because of these design features, the second aspect of the invention hashad the following problems.

(a) The pH of the effluent from the anode compartment has to be loweredto a value that induces oil-water separation, namely, a value thatcauses the emulsified oil particles to be adequately ruptured. If thedesired oil-water separation can be accomplished at higher pH levels,the separation efficiency will be improved.

(b) As the treatment progresses, the diaphragm and the surface of eachelectrode are gradually contaminated and, as a result, less water willpermeate through the diaphragm to move from the anode to the cathodecompartment, thereby increasing the electrolytic voltage required tomaintain a constant electrolytic current level. It is thereforenecessary to provide means of decontaminating the diaphragm andelectrodes to thereby restore the desired permeate flow and voltage.

Hence, the third aspect of the invention intends to provide a method andan apparatus for oil-water separation of wastewater containing anemulsified oil that are capable of the intended oil-water separation ata higher pH than in the second aspect and which adopt simple means ofdecontaminating the diaphragm and electrodes to thereby restore thedesired permeate flow and voltage.

The third aspect of the invention relates to a method for oil-waterseparation of wastewater containing a surfactant and an oil content thathas been emulsified by the action of said surfactant. As in the firstaspect, the diaphragm electrolyzer used in the third aspect has an anodeand a cathode provided in an anode compartment and a cathodecompartment, respectively, which are spaced apart by a porous diaphragm.As in the second aspect, the anode compartment of the diaphragmelectrolyzer is supplied with the feed wastewater for electrolysis andpart of the electrolyzed wastewater is passed through the diaphragm toenter the cathode compartment, from which it is discharged while theremainder of the electrolyzed wastewater is discharged from the anodecompartment and thence introduced into a layer packed with an adheringmaterial, where it is brought into contact with the adhering materialand, thereafter, the effluent is directed to an oil-water separationstep for accomplishing the intended oil-water separation.

In a preferred embodiment, the polarities of the two electrodes arechanged alternately at specified time intervals during the step ofdiaphragm electrolysis such that the anode compartment is switched tothe cathode compartment and vice versa.

According to the third aspect of the invention, there is also providedan apparatus for water-oil separation of wastewater containing asurfactant and an oil content that has been emulsified by the action ofsaid surfactant, which comprises a diaphragm electrolyzer having ananode and a cathode provided in an anode compartment and a cathodecompartment, respectively, that are spaced apart by a porous diaphragm,a column packed with an adhering material, and a tank for oil-waterseparation of the effluent from the anode compartment, with thediaphragm electrolyzer, the packed column and the oil-water separationtank being interconnected in such a way that the separation tank issupplied with the feed wastewater, which is withdrawn from the bottom ofsaid separation tank to be fed into the anode compartment of saiddiaphragm electrolyzer, with part of the effluent from the anodecompartment being passed through the diaphragm to enter the cathodecompartment, from which it is discharged while the remainder of theeffluent is directed in an upward flow into the packed column, where itis brought into contact with the adhering material and, thereafter, saidremainder is returned to the oil-water separation tank.

In a preferred embodiment, the diaphragm electrolyzer is adapted to becapable of changing the polarities of the two electrodes alternatelysuch that the anode is switched to the cathode compartment and viceversa while, at the same time, the pipes connecting to the anode andcathode compartments are accordingly switched.

Since the third aspect of the invention is similar to the second aspectin many ways, the following description concerns only the difference,which is the provision of a coalescer, or a layer packed with anadhering material, between the diaphragm electrolyzer and the tank foroil-water separation of the effluent from the anode compartment. In thesecond aspect, the wastewater to be treated which contains an emulsifiedoil is supplied into the tank for oil-water separation of the effluentfrom the anode compartment and it is then withdrawn from the bottom ofsaid separation tank to be fed into the anode compartment of thediaphragm electrolyzer and part of the wastewater electrolyzed in theanode compartment is passed through the diaphragm to enter the cathodecompartment, from which it is discharged where the remainder of theelectrolyzed wastewater is returned to the associated separation tank.In the third aspect, the electrolyzed wastewater in the anodecompartment is partly returned to the associated separation tank afterit is directed in an upward flow into the layer packed with an adheringmaterial such that it is brought into contact with said adheringmaterial.

By causing the oil particles in the electrolyzed wastewater to attach tothe adhering material, the oil particles will collide at an increasedrate and their coalescing and coarsening are sufficiently promoted toprovide the so-called "oil-water separation effect" of the coalescer.This effect combines synergistically with the oil-water separationeffect of diaphragm electrolysis to increase the specified pH foroil-water separation and thereby improve the overall efficiency ofoil-water separation.

The layer packed with the adhering material which works as a coalescermay assume a cylindrical, rectangular or any other shape; the adheringmaterial with which the layer is to be packed may be of any nature aslong as it is chemically stable and has a large specific surface area,and an advantageous example of its shape is an aggregate of particulatematter or fibers. Exemplary particulate adhering materials includequartz sand, zeolite and kaolin, and exemplary fibrous adheringmaterials include slag wool and synthetic fibers. The larger the volumeof the layer packed with the adhering material, the longer the contacttime and the more effective the layer is as a coalescer. The superficialvelocity (SV in h⁻¹) of the electrolyzed wastewater passing through thelayer packed with the adhering material will suffice if it is within therange of 20-200 h⁻¹.

As an apparatus for wastewater treatment, the additional advantage ofcausing the oil particles to attach to the adhering material is thatthey will have an increased density on the adhering material to achievemore efficient contact between the oil content and the electrolyzedwastewater. As a result, hydrophobic organic matter, such as thenonionic surfactant, in the electrolyzed wastewater or the anionicsurfactant which has become no longer dissociable into an acid onaccount of the acidic nature of the electrolyzed wastewater can beextracted with the oil content and, hence, can be rejected together withthe latter.

In a preferred embodiment of the third aspect, the polarities of the twoelectrodes are changed alternately at specified time intervals such thatthe anode is switched to the cathode and vice versa at specified timeintervals while, at the same time, the pipes connecting to the anode andcathode compartments are accordingly switched. The wastewater in theanode compartment is usually acidic in nature, so part of the anionicsurfactant will become insoluble and the resulting insoluble matter, aswell as the broken oil particles that are no longer in an emulsifiedstate will occasionally form a deposit that contaminates the surfaces ofthe electrodes and the diaphragm. On the other hand, the wastewater inthe cathode compartment is alkaline, so insoluble hydroxides of alkalineearth metals may be generated to form a deposit that adheres to thesurfaces of the electrodes and the diaphragm.

If electrode polarity is changed after the lapse of a certain time oftreatment when the above-described contamination has progressed to someextent, the wastewater in the anode compartment which has so far beenacidic turns alkaline whereas the wastewater in the cathode compartmentwhich has been alkaline turns acidic and, as a consequence, theinsoluble contaminants in the respective compartments will be dissolvedaway to thereby restore the permeate flow through the diaphragm and theelectrolytic voltage. The shorter the interval between successivechanges of polarity, the more effective the polarity change is inremoving the contaminants. On the other hand, frequency polarity changeswill cause the disadvantage of shortening the electrode life. Dependingon the contaminant loading of the wastewater to be treated (which varieswith the concentrations of the anionic surfactant, the oil content,alkaline earth metals, etc.), the interval of polarity changes may bedetermined within the range of 4-72 h.

The wastewater to be treated in the third aspect of the invention andthe surfactants to be present in the wastewater are identical to thosedescribed in connection with the second aspect.

The mechanism by which the wastewater containing an emulsified oil istreated in the anode compartment of the diaphragm electrolyzer, as wellas the effect of electrical permeation that is utilized to enrich theoil content are also identical to those described in connection with thesecond aspect.

The pH adjusted effluent from the anode compartment is returned to theanode compartment in the same manner as in the second aspect.

The diaphragm electrolyzer and the porous diaphragm which are to beemployed in the third aspect of the invention are identical to thoseused in the first aspect.

An example of the apparatus for performing oil-water separationaccording to the third aspect of the invention is shown in FIG. 3. Areceptacle 1 holds wastewater containing an emulsified oil content. Thewastewater is fed into an oil-water separation tank 2 via an inlet 5.The tank 2 is equipped with an electric heater 3 and an air vent valve4. The wastewater emerging from the tank 2 via an outlet 6 is forced bya feed pump 9 to pass through a prefilter 24 into a diaphragmelectrolyzer 13 at an anode compartment 15 containing an anode 14. Thediaphragm electrolyzer 13 is divided into the anode compartment 15 and acathode compartment 17 by a diaphragm 18 and they respectively containthe anode 14 and a cathode 16, which are supplied with a predetermineddc current from a power source 19. In the anode compartment 15, water iselectrolyzed to generate hydrogen ions, which will cause the loss of thesurface activity of the anionic surface active substance such as thesodium alkylsulfonate in the wastewater which has been fed into saidanode compartment.

The wastewater thus rendered acidic emerges from the anode compartment15 of the diaphragm electrolyzer 13 to pass through a column 25 packedwith an adhering material and enter the oil-water separation tank 2 viaan inlet 7; in the separation tank 2, the effluent is heated with theelectric heater 3 to a temperature higher than the cloud point of thenonionic surfactant.

The effluent fed into the oil-water separation tank 2 needs only to beheated to a temperature in the range of 50°-80° C., preferably 60°-70°C., so that the oil content is selectively separated by flotation.

The effluent is allowed to stay within the oil-water separation tank 2for a detention time of no more than about 20 minutes and the separatedoil content is continuously or intermittently discharged out of thesystem whereas the aqueous phase is recovered from the tank 2 via theoutlet 6 to return to the anode compartment 15, thereby accomplishingthe intended oil-water separation.

The electrolyzed wastewater flowing from the anode compartment 15 pastthe diaphragm 18 to enter the cathode compartment 17 will emergetherefrom and passes through a valve 31 to be returned to the receptacle1.

If these treatments are performed for a certain time period, thesurfaces of the diaphragm and the electrodes will be graduallycontaminated and, as a result, less water will permeate through thediaphragm to move from the anode to the cathode compartment, therebyincreasing the electrolytic voltage required to maintain a constantelectrolytic current level. If this condition occurs, a switching board32 connected to the power source 19 is operated to change the polarityof the voltage applied to the electrolyzer 13 in such a way that theanode compartment 15 is switched to cathode compartment and the cathodecompartment 17 to anode compartment while, at the same time, valves 26,28 and 31 are closed whereas valves 27, 29 and 30 are opened. As aresult, the contaminants are removed from the surfaces of the diaphragmand the electrodes to thereby restore the permeate flow and theelectrolytic voltage.

(4) Fourth Aspect

In FIG. 3 showing the apparatus for performing oil-water separationaccording to the third aspect of the invention, the effluent(electrolyzed wastewater) from the anode compartment 15 passes throughthe packed column 25 to be returned to the oil-water separation tank 2.

As is well known, the faster the flow rate of the wastewater on theanode side of the porous membrane, the smaller the likelihood ofmembrane contamination and, as a result, one can retard the increase inthe electrolytic voltage while preventing the drop in the permeate flux.In this respect, it is advantageous to increase the quantity of the feedto the anode compartment.

However, if the quantity of the feed to the anode compartment isincreased in the oil-water separation system according to the thirdaspect of the invention, not only the time of contact in the packedcolumn but also the time of detention in the tank for oil-waterseparation of the effluent from the anode compartment will decreaseunavoidably and this in turn will interfere with the two phenomenadesirable for the invention, i.e., the coalescing and coarsening of theoil particles in the packed column, and the floating of the oil contentin the tank for oil-water separation of the effluent from the anodecompartment.

Hence, the fourth aspect of the invention intends to provide a methodand an apparatus for oil-water separation of wastewater containing anemulsified oil that are capable of increasing the flow rate of thewastewater on the anode side of the diaphragm without increasing thequantity of the feed to the packed column or the oil-water separationtank for the effluent from the anode compartment.

The fourth aspect of the invention relates to a method for oil-waterseparation of wastewater containing a surfactant and an oil content thathas been emulsified by the action of said surfactant. As in the firstaspect, the diaphragm electrolyzer used in the fourth aspect has ananode and a cathode provided in an anode compartment and cathodecompartment, respectively, which are spaced apart by a porous diaphragm.As in the third aspect, the anode compartment of the diaphragmelectrolyzer is supplied with the feed wastewater for electrolysis andpart of the electrolyzed wastewater is passed through the diaphragm toenter the cathode compartment, from which it is discharged while theremainder of the electrolyzed wastewater is discharged from the anodecompartment. In the fourth aspect, the electrolyzed wastewaterdischarged from the anode compartment is introduced into theintermediate portion of a gas-liquid separator and part of the influentis withdrawn from the top of the gas-liquid separator and introducedinto a layer packed with an adhering material, where it is brought intocontact with the adhering material and, thereafter, the effluent isdirected to an oil-water separation step for accomplishing the intendedoil-water separation while, at the same time, the remainder is withdrawnfrom the bottom of the gas-liquid separator to be returned to mix withthe feed to the electrolysis step.

In a preferred embodiment, the polarities of the two electrodes arechanged alternately at specified time intervals during the step ofdiaphragm electrolysis such that the anode compartment is switched tothe cathode compartment and vice versa.

According to the fourth aspect of the invention, there is also providedan apparatus for oil-water separation of wastewater containing asurfactant and an oil content that has been emulsified by the action ofsaid surfactant, which comprises a diaphragm electrolyzer having ananode and a cathode provided in an anode compartment and a cathodecompartment, respectively, that are spaced apart by a porous diaphragm,a gas-liquid separator, a column packed with an adhering material, and atank for oil-water separation of the effluent from the anodecompartment, with the diaphragm electrolyzer, the gas-liquid separator,the packed column and the oil-water separation tank being interconnectedby channels in such a way that the separation tank is supplied with thefeed wastewater, which is withdrawn from the bottom of said separationtank to be fed into the anode compartment of said diaphragmelectrolyzer, with part of the electrolyzed wastewater being passedthrough the diaphragm to enter the cathode compartment, from which it isdischarged while the remainder of the electrolyzed wastewater isdischarged from the anode compartment and thence introduced into theintermediate portion of the gas-liquid separator and a part of theinfluent is withdrawn from the top of the gas-liquid separator andintroduced in an upward flow into the packed column, where it is broughtinto contact with the adhering material and, thereafter, the effluent isreturned to the oil-water separation tank while, at the same time, theremainder is withdrawn from the bottom of the gas-liquid separator to bedirectly returned to the feed channel to the anode compartment.

In a preferred embodiment, the diaphragm electrolyzer is adapted to becapable of changing the polarities of the two electrodes alternatelysuch that the anode is switched to the cathode compartment and viceversa while, at the same time, the pipes connecting to the anode andcathode compartments are accordingly switched.

Since the fourth aspect of the invention is similar to the third aspectin many ways, the following description concerns only the difference,which is the provision of a gas-liquid separator between the diaphragmelectrolyzer and the column packed with an adhering material.

In the fourth aspect, the effluent from the anode compartment of thediaphragm electrolyzer is introduced into the intermediate portion ofthe gas-liquid separator and part of the influent is withdrawn from thetop of the gas-liquid separator to be fed into the packed column whereasthe remainder is withdrawn from the bottom of the separator to bereturned to a suction port of a feed pump. In this way, the flow rate ofthe feed wastewater on the anode side of the diaphragm can be increasedwithout increasing the quantity of the feed to either the packed columnor the oil-water separation tank for the effluent from the anodecompartment. The gas-liquid separator has the added advantage ofproviding an oil floating action due to the tiny oxygen bubbles in thewastewater which have been generated by electrolysis. Yet anotheradvantage of the fourth aspect of the invention is that in order toincrease the throughput of the overall system, one needs only toincrease the sizes of the electrolyzer and the feed pump and there is noneed to revamp the packed column and the oil-water separation tank forthe effluent from the anode compartment.

If, on the other hand, part of the effluent from the anode compartmentis simply returned to the suction port of the feed pump without passingthrough the gas-liquid separator, oxygen bubbles will accumulate andgrow in the anode compartment to such a large size that the feed flowrate and the voltage become unstable.

The influent into the gas-liquid separator has desirably the longestpossible detention time but 0.5-2 min will normally suffice. If desired,the gas-liquid separator may be fitted with a perforated plate forpromoting the intended gas-liquid separation.

In the fourth aspect of the invention, the column packed with anadhering material is provided as a coalescer between the gas-liquidseparator and the oil-water separation tank for the effluent from theanode compartment. The electrolyzed wastewater withdrawn from the top ofthe gas-liquid separator is returned to the associated oil-waterseparation tank after it is directed in an upward flow into the packedcolumn such that it is brought into contact with the adhering material.

The purpose of causing the oil particles in the electrolyzed wastewaterto attach to the adhering material and the advantages obtained werealready described in connection with the third aspect.

The packed column to be used as a coalescer and its function were alsodescribed in connection with the third aspect.

As in the third aspect, the polarities of the two electrodes may bechanged alternately and the resulting advantages are also the same asdescribed in connection with the third aspect.

The wastewater to be treated in the fourth aspect and the surfactants tobe present in the wastewater are also identical to those described inconnection with the third aspect.

The pH adjusted effluent from the anode compartment is returned to theanode compartment in the same manner as in the second aspect.

The diaphragm electrolyzer and the porous diaphragm which are to beemployed in the fourth aspect are also identical to those used in thefirst aspect.

An example of the apparatus for performing oil-water separationaccording to the fourth aspect of the invention is shown in FIG. 5. Areceptacle 1 holds wastewater containing an emulsified oil content. Thewastewater is fed into an oil-water separation tank 2 via an inlet 5.The tank 2 is equipped with an electric heater 3 and an air vent valve4. The wastewater emerging from the tank 2 via an outlet 6 is forced bya feed pump 9 into a diaphragm electrolyzer 13 at an anode compartment15 containing an anode 14. The diaphragm electrolyzer 13 is divided intothe anode compartment 15 and a cathode compartment 17 by a diaphragm 18and they respectively contain the anode 14 and a cathode 16, which aresupplied with a predetermined dc current from a power source 19. In theanode compartment 15, water is electrolyzed to generate hydrogen ion,which will cause the loss of the surface activity of the anionic surfaceactive substance such as the sodium alkylsulfonate in the wastewaterwhich has been fed into said anode compartment.

The wastewater thus rendered acidic emerges from the anode compartment15 of the diaphragm electrolyzer 13 to be introduced into theintermediate portion of a gas-liquid separator 33 and part of theinfluent is withdrawn from the top and passed through a column 25 packedwith an adhering material and thence supplied into the oil-waterseparation tank 2 via an inlet 7; in the separation tank 2, the effluentis heated with the electric heater 3 to a temperature higher than thecloud point of the nonionic surfactant. The remainder of the influentinto the gas-liquid separator 33 is withdrawn from the bottom andreturned to the suction port of the feed pump 9.

The effluent fed into the oil-water separation tank 2 needs only to beheated to a temperature in the range of 50°-80° C., preferably 60°-70°C., so that the oil content is selectively separated by flotation.

The effluent is allowed to stay within the oil-water separation tank 2for a detention time of no more than about 20 minutes and the separatedoil content is continuously or intermittently discharged out of thesystem whereas the aqueous phase is recovered from the tank 2 via theoutlet 6 to return to the anode compartment 15.

The electrolyzed wastewater flowing from the anode compartment 15 pastthe diaphragm 18 to enter the cathode compartment 17 will emergetherefrom and passes through a valve 31 to be returned to the receptacle1.

If these treatments are performed for a certain time period, thesurfaces of the diaphragm and the electrodes will be graduallycontaminated and, as a result, less water will permeate through thediaphragm to move from the anode to the cathode compartment, therebyincreasing the electrolytic voltage required to maintain a constantelectrolytic current level. If this condition occurs, a switching board32 connected to the power source 19 is operated to change the polarityof the voltage applied to the electrolyzer 13 in such a way that theanode compartment 15 is switched to cathode compartment and the cathodecompartment 17 to anode compartment while, at the same time, valves 26,28 and 31 are closed whereas valves 27, 29 and 30 are opened. As aresult, the contaminants are removed from the surfaces of the diaphragmand the electrodes to thereby restore the permeate flow and theelectrolytic voltage.

The following examples are provided for the purpose of furtherillustrating the four aspects of the present invention but are in no wayto be taken as limiting.

EXAMPLE 1

An apparatus for oil-water separation was constructed according to thedesign of the first aspect of the invention shown in FIG. 1 and it wasused in testing the oil-water separation of a cleaning solution. Thediaphragm electrolyzer was a sealed rectangular type made of poly(vinylchloride). The anode was a titanium plate with an area of 0.1 m² havinga platinum plate deposited thereon and so was the cathode. The diaphragmwas a MF membrane made of an organic synthetic polymer. The oil-waterseparation tank had a capacity of 36 L with a built-in electric heaterof 1 kw.

A detergent was prepared from an aqueous solution containing 15% of anonionic surfactant of a polyoxyethylene alkyl phenyl ether type havinga cloud point of 49° C. and 3.5% of sodium sulfate. In the cleaningtank, the detergent was diluted with water to a concentration of 5% andhad the pH adjusted with sodium hydroxide to 8.7, thereby preparing acleaning solution of 200 L. The cleaning solution was used with itstemperature controlled at 45° C.

An emulsifiable water-insoluble rust preventive oil was used in thetest. This oil contained a large amount of a barium alkylsulfonate as arust inhibitor.

Before starting the test for oil-water separation, 4 L of the rustpreventive oil was charged into the cleaning tank and diluted with thecleaning solution to a concentration of 2%. In order to form a stableemulsion of the oil in the cleaning solution, the latter was agitatedwith a cascaded pump (3,600 rpm) for 30 min at a flow rate of 15 L/min.

Details of the testing procedure and conditions, as well as the testresults are given below.

The cleaning solution in the cleaning tank was pumped into the anodecompartment of the diaphragm electrolyzer at a flow rate of 200 L/h. Theelectrolytic current was set at 1.2 A such that the pH of the cleaningsolution emerging from the anode compartment would be in the range of5.0-5.5. The corresponding electrolytic voltage was about 6 V.

The cleaning solution exiting from the anode compartment was introducedinto the oil-water separation tank, where it was heated to effectoil-water separation. The temperature setting of the built-in heater inthe separation tank was 65° C. The oil-freed cleaning solution was sentto the cathode compartment of the diaphragm electrolyzer. The cleaningsolution exiting from the cathode compartment was returned to thecleaning tank. The cleaning solution had a pH of 8.8 as it emerged fromthe cathode compartment.

Every two hours after the start of the test, the oil collecting in theupper part of the oil-water separation tank was discharged out of thesystem and the quantity of the oil thus discharged on each occasion wasmeasured. The time-dependent profiles of the cumulative oil dischargedand the oil concentration of the cleaning solution calculated from thecumulative oil discharged are shown in Table 1 below.

                  TABLE 1    ______________________________________    Example 1           Example 2                      Oil               Oil            Cumulative                      Concen-   Cumulative                                        concen-    Time,   oil dis-  tration   oil dis-                                        tration,    h       charge, L %         charge, L                                        %    ______________________________________    0       0         2.0       0        2.05    2       1.18      1.4       0.91    1.6    4       2.12      0.9       1.75    1.2    6       2.98      0.5       2.50    0.8    8       3.75      0.1       2.98    0.6    10      4.13      --        3.36    0.4    12      4.42      --        3.49    0.3    ______________________________________

As one can see from Table 1, effective oil-water separation could beaccomplished in accordance with the first aspect of the invention.

After 10 h of the testing, the cumulative oil discharge exceeded 4 L,indicating that part of the nonionic surfactant in the cleaning solutionwas also separated by flotation together with the oil.

In order to demonstrate the effectiveness of diaphragm electrolysisperformed in accordance with the first aspect of the invention, thefollowing comparative test was made.

Comparative Example 1

An apparatus for oil-water separation of the same design as used inExample 1 was applied, except that the power supply to the diaphragmelectrolyzer was turned off such that no electrolytic treatment would beperformed. A test for oil-water separation was conducted using the samecleaning solution and rust preventive oil as employed in Example 1. Whenthe cleaning solution having the rust preventive oil dispersed thereinwas not electrolyzed, no oil-water separation occurred in the cleaningsolution.

EXAMPLE 2

A detergent was prepared rom an aqueous solution containing 15% of anonionic surfactant of a polyoxyethylene alkyl phenyl ether type havinga cloud point of 49° C., 6% of a nonionic surfactant of a pluronic typehaving a cloud point of 29° C. and 3.5% of sodium sulfate. In thecleaning tank, the detergent was diluted with water to a concentrationof 5% and had the pH adjusted with sodium hydroxide to 9.7, therebypreparing a cleaning solution of 200 L. The cleaning solution was usedwith its temperature controlled at 40° C.

A mixture of a water-soluble cutting oil of type W1, an emulsifiablewater-insoluble rust preventive oil and a non-emulsifiablewater-insoluble cutting oil was used in the test. The water-solublecutting oil, the rust preventive oil and the water-insoluble cutting oilwere added at respective concentrations of 0.1%, 1.0% and 1.0%. Thewater-insoluble cutting oil had a small content of an anionic surfaceactive substance; in contrast, the water-soluble cutting oil containedabout 25% of an anionic surfactant (e.g. sodium alkylsulfonate or sodiumalkylcarboxylate) and its mineral oil content was about 60%.

The testing equipment and procedure employed in Example 2 were identicalto those used in Example 1; however, in order to ensure that thecleaning solution existing from the anode compartment would have a pH of5.0-5.5, the electrolytic current was set at 10 A. The correspondingelectrolytic voltage was about 20 V. The cleaning solution had a pH of9.9 as it emerged from the cathode compartment.

The results of the test conducted in Example 2 are also shown in Table1, from which one can see that effective oil-water separation wasaccomplished in accordance with the first aspect of the invention. InExample 2, the nonionic surfactant in the cleaning solution was notfound to be separated by flotation together with the oil.

In order to demonstrate the effectiveness of diaphragm electrolysisperformed in Example 2 according to the first aspect of the invention,the following comparative test was made.

Comparative Example 2

An apparatus for oil-water separation of the same design as used inExample 2 was applied, except that the power supply to the diaphragmelectrolyzer was turned off such that no electrolytic treatment would beperformed. A test for oil-water separation was conducted on a cleaningsolution in which the same water-soluble cutting oil, rust preventiveoil and water-insoluble cutting oil as used in Example 2 were dispersed.

When the cleaning solution having these three oils dispersed therein wasnot electrolyzed, no oil-water separation occurred in the cleaningsolution.

According to the first aspect of the invention, the following advantagesare obtained.

1) The method for oil-water separation of cleaning solutions accordingto the first aspect of the invention can effectively be used inseparating an oil from water in a wide scope of cleaning solutionsranging from simple water to mixtures of water and nonionic or anionicsurfactants.

2) The method of adding nonionic surfactants to cleaning solutions andheating them to temperatures higher than the cloud points of thesurfactants is generally held to be effective in achieving oil-waterseparation of cleaning solutions containing oily contaminants; however,this method becomes ineffective if the cleaning solutions also containanionic surfactants such as sodium alkylsulfonates which are heavilyincorporated in water-soluble oils or anionic surface active substancessuch as calcium sulfonate which are incorporated in water-insolubleoils; in contrast, the method according to the first aspect of theinvention is applicable under such adverse conditions and exhibitsmarked performance in oil-water separation.

3) The method according to the first aspect of the invention has no needto use violent chemicals such as acids and alkalies.

4) The method is capable of preventing the putrefaction of cleaningsolutions even if no chemical sterilizers are added.

5) The apparatus for oil-water separation of cleaning solutionsaccording to the first aspect of the invention is not costly, is simpleto operate and guarantees consistent operation.

EXAMPLE 3

An apparatus for oil-water separation was constructed according to thedesign of the second aspect of the invention shown in FIG. 2 and it wasused in testing the oil-water separation of wastewater containing anemulsified oil. The diaphragm electrolyzer was a sealed rectangular typemade of polypropylene resin. Each of the anode and the cathode was aPt-plated Ti electrode having an effective area of 0.1 m². The diaphragmwas a MF membrane made of an organic synthetic polymer having a nominalpore size of 0.5 μm. The tank for oil-water separation of the effluentfrom the anode compartment had a capacity of 36 L with a built-inelectric heater of 1 kw. The tank for oil-water separation of theeffluent from the cathode compartment had a capacity of 10 L.

Liquid waste coolant was used in the test as the wastewater containingan emulsified oil. The liquid waste coolant was prepared by 30-folddilution of a water-soluble cutting oil of type W1 and had an oilconcentration of 1.8% and a pH of 8.8. After adding 0.05% of sodiumtripolyphosphate to it, the liquid waste was first charged into thereceptacle 1 from which it was injected into the tank 2 for oil-waterseparation of the effluent from the anode compartment. The injection ofthe liquid waste was 240 L.

The liquid waste was electrolyzed at a current of 20 A as it was forcedby pump 9 to circulate between the anode compartment 15 of theelectrolyzer 13 and the separation tank 2 at a flow rate of 8 L/min. Thecorresponding electrolytic voltage was about 56 V. The temperaturesetting of the electric heater 3 in the separation tank 2 was 55° C. Theeffluent from the cathode compartment 17 flowed at a rate of 0.32 L/minand had a pH of 11.3 and an oil concentration of 0.06%. The effluent wasthen returned to the receptacle 1.

When the electrolysis started, the pH of the effluent from the anodecompartment declined gradually and it was 4.5 after the lapse of 2.2 h.Then, valve 12 was operated to have the effluent return continuously tothe receptacle 1 such that its flow rate would be 0.15 L/min.Thereafter, the pH of the effluent stabilized in the neighborhood of4.5. The effluent was found have an oil concentration of 0.3% at theoutlet 6.

The time-dependent profile of the oil concentration of the liquid wastein the receptacle 1 is shown in Table 2. After the lapse of 12 h, thetest was stopped and the oil was withdrawn from the separation tank 2and the oil-separation tank 20 for the effluent from the cathodecompartment via respective oil drain valves and a total of about 3.1 Lof the oil was recovered. Thus, one may safely conclude that effectiveoil-water separation of the liquid waste coolant could be accomplishedin accordance with the second aspect of the invention.

Comparative Example 3

In order to further clarify the effectiveness of the second aspect ofthe invention, a comparative test for oil-water separation was made withthe same liquid waste by means of the same apparatus as in Example 3,except that the power supply to the electrolyzer was not turned on toperform an electrolytic treatment. In this case, no oil-water separationoccurred in the liquid waste.

EXAMPLE 4

The second aspect of the invention was applied to treat wastewatercontaining an emulsified oil, which was an oil-containing cleaningsolution of the type described below. A detergent was prepared from anaqueous solution containing 20% of a nonionic surfactant of apolyoxyethylene alkyl phenyl ether type having a cloud point of 49° C.and 3.0% of sodium sulfate. A 5% aqueous solution of the detergent wasprepared in the receptacle 1 and adjusted to have a temperature of 45°C. and its pH adjusted to 9.8 with sodium tripolyphosphate and the thusprepared cleaning solution was used in a test for oil-water separation.The oil to be separated was an emulsifiable water-insoluble rustpreventive oil. This oil contained a large amount of a bariumalkylsulfonate as a rust inhibitor. Four liters of the oil was chargedinto the cleaning tank to give an initial oil concentration of 2%. Inorder to form a stable emulsion of the oil in the cleaning solution, thelatter was agitated with a cascaded pump (3,600 rpm) for 30 min at aflow rate of 15 L/min.

The testing equipment and procedure employed in Example 4 were identicalto those used in Example 3, except that electrolysis was performed at acurrent of 25 A; the corresponding electrolytic voltage was about 29 V.The temperature setting of the electric heater 3 in the tank 2 foroil-water separation of the effluent from the anode was 65° C. Theeffluent from the cathode compartment 17 flowed at a rate of 0.28 L/minand had a pH of 11.5 and an oil concentration of 0.15%.

When the electrolysis started, the pH of the effluent from the anodecompartment declined gradually and it was 2.5 after the lapse of 3 h.Then, valve 12 was operated to have the effluent return continuously tothe receptacle 1 such that its flow rate would be 0.1 L/min. Thereafter,the pH of the effluent stabilized in the neighborhood of 2.5. Theeffluent was found to have an oil concentration of 0.6% at the outlet 6.

The time-dependent profile of the oil concentration of the cleaningsolution in the receptacle 1 is shown in Table 2. After the lapse of 12h, the test was stopped and the oil was withdrawn from the separationtanks 2 and 20 via respective oil drain valves and a total of about 3.5L of the oil was recovered, indicating that part of the nonionicsurfactant in the cleaning solution was also separated by flotationtogether with the oil.

Thus, one may safely conclude that effective oil-water separation of thecleaning solution could be accomplished in accordance with the secondaspect of the invention.

Comparative Example 4

In order to further clarify the effectiveness of the diaphragmelectrolysis treatment according to the second aspect of the invention,a comparative test of oil-water separation was made with the samecleaning solution by means of the same apparatus as in Example 4, exceptthat the power supply to the electrolyzer was not turned on to performan electrolytic treatment. In this case, no oil-water separationoccurred in the cleaning solution.

                  TABLE 2    ______________________________________             Example 3  Example 4    Time, h    Oil concentration, %                            Oil concentration, %    ______________________________________    0          1.8          2.0    2          1.5          1.7    4          1.1          1.4    6          0.8          1.2    8          0.6          1.0    10         0.5          0.9    12         0.4          0.8    ______________________________________

According to the second aspect of the invention, continuous oil-waterseparation can be accomplished at low cost and with high efficiency on awide variety of cleaning solutions including not only those contaminatedby non-emulsifiable water-insoluble oils typified by press working oilsand rolling mill oils but also those contaminated by water-soluble oilscontaining anionic or nonionic surfactants, as well as thosecontaminated by emulsifiable water-insoluble oils incorporating anionicsurface active substances. The invention is also applicable to theoil-water separation of wastewater containing an emulsified oil asexemplified by water-soluble, liquid waste cutting oils and coolants.

EXAMPLE 5

An apparatus for oil-water separation was constructed according to thedesign of the third aspect of the invention shown in FIG. 3 and it wasused in testing the oil-water separation of wastewater containing anemulsified oil. The diaphragm electrolyzer was a sealed rectangular typemade of polypropylene resin. Each of the anode and the cathode was aPt-plated Ti electrode having an effective area of 0.1 m². The diaphragmwas a MF membrane made of an organic synthetic polymer having a nominalpore size of 0.5 μm. The layer packed with an adhering material wasprovided from ADVANTEC CORP. as a filter cartridge in the form of apincushion (nominal filtering precision, 5 μm; filter medium made of PPSfibers), and the spatial velocity (SV) was 100 h⁻. The tank foroil-water separation of the effluent from the anode compartment had acapacity of 36 L.

The wastewater containing an emulsified oil which was to be treated inExample 5 was warm cleaning water containing a water-soluble cutting oilof type W1. This cleaning water was subjected to continuous oil-waterseparation in the following manner. First, city water having 0.02% ofsodium tripolyphosphate added was charged into the cleaning tank 1 andthe oil-water separation tank 2 for the effluent from the anodecompartment. The total water charge was 240 L. The water had a pH of 8.9and it was held at 50° C. by means of the electric heater built in thecleaning tank 1. In the next step, a water-soluble cutting oil of typeW1, No. 1 available from company X was injected continuously into thecleaning tank 1 at a flow rate of 1.3 mL/min. The cleaning water in thetank 1 was stirred with an agitator to disperse the water-solublecutting oil.

The cleaning water thus prepared was forced by the pump 9 to circulatein a loop consisting of the anode compartment 15 of the electrolyzer 13,the packed layer 25 and the separation tank 2 at a flow rate of 8 L/minwhile electrolysis was conducted at a constant current of 25 amperes. Atthe start of the electrolysis, the electrode 14 was the anode whereasthe electrode 16 was the cathode, with valves 26, 28 and 31 being fullyopen but valves 27, 29 and 30 fully closed. The effluent from thecathode compartment was returned to the cleaning tank 1 at a flow rateof 0.42 L/min with the pH being in the neighborhood of 11.0.

When the electrolysis started, the pH of the effluent from the anodecompartment declined gradually and it was 6.0 after the lapse of 1 h.Then, valve 12 was operated to have the effluent return continuously tothe cleaning tank 1 such that its flow rate would be 0.45 L/minThereafter, the pH of the effluent stabilized in the neighborhood of6.0.

Every 12 hours of the electrolytic treatment, the polarities of the twoelectrodes were altered. On the first occasion, the output polarity ofthe dc power supply was changed and, at the same time, the electrode 14was switched to cathode whereas the electrode 16 was switched to anode,with the valves 26, 28 and 31 being rendered fully closed and the valves27, 29 and 30 fully open. Subsequent polarity changes were made byreversing the procedure of the previous operation.

At intervals of several hours in the continuous test, the oil separatedby flotation in the separation tank 2 was discharged and analysis of theoil concentration and TOC (total organic carbon concentration) of thecleaning water in the tank 1 was carried out. In addition, theelectrolytic voltage was recorded for each analysis.

FIGS. 4a and 4b show the time-dependent profiles of the oilconcentration and TOC of the cleaning water, and FIG. 4c shows theprofile of electrolytic voltage. Obviously, the oil-water separation ofthe cleaning water containing the water-soluble cutting oil could beeffectively accomplished in accordance with the third aspect of theinvention and one may safely conclude that the electrolytic voltageremained fairly constant.

Comparative Example 5

In order to further clarify the effectiveness of the third aspect of theinvention, a comparative test for oil-water separation was made with thesame apparatus and under the same conditions as in Example 5, exceptthat the packed layer 25 was not provided and that no polarity changeswere effected. The data of the comparative test are also shown in FIGS.4a-4c. Obviously, in the absence of the packed layer, no effectiveoil-water separation could be accomplished at a fairly high pH (6.0 inthe comparative case). In order to perform effective oil-waterseparation of the cleaning water in the absence of the packed layer 25,the return flow to the cleaning tank 1 had to be reduced while, at thesame time, the pH of the effluent from the anode compartment had to belowered to 4.5.

The electrolytic voltage which was about 30 volts at the start of theelectrolysis increased gradually and 48 h later, it became as high as 95volts. Since the temperature of the effluent from the cathodecompartment at that time was 85° C. which was close to the heatresisting limit of the electrolyzer, the test was suspended.

According to the third aspect of the invention, the following advantagesare obtained.

a) The packed layer with an adhering material is provided as a coalescerbetween the anode compartment of the diaphragm electrolyzer and the tankfor oil-water separation of the effluent from the anode compartment andthis is effective in raising the pH for oil-water separation and therebyachieving improved performance in oil-water separation.

b) Since the electrolyzed wastewater makes highly efficient contact withthe oil content in the packed layer, the hydrophobic organic matter inthe electrolyzed wastewater can be extracted with the oil content and,hence, can be rejected out of the system together with the latter.

c) By changing the polarities of the two electrodes alternately atspecified time intervals, the permeate flow from the anode to thecathode compartment, as well as the electrolytic voltage required tomaintain a contact current level can be held within specified ranges fora prolonged time period.

EXAMPLE 6

An apparatus for oil-water separation was constructed according to thedesign of the fourth aspect of the invention shown in FIG. 5 and it wasused in testing the oil-water separation of wastewater containing anemulsified oil. The diaphragm electrolyzer was a sealed rectangular typemade of polypropylene resin. Each of the anode and the cathode was aPt-plated Ti electrode having an effective area of 0.2 m². The diaphragmwas a MF membrane made of an organic synthetic polymer having a nominalpore size of 0.5 μm. The packing material was provided from ADVANTECCORP. as a filter cartridge in the form of a pincushion (nominalfiltering precision, 5 μm; filter medium made of PPS fibers), and thespatial velocity (SV) was 100 h⁻¹. The tank for oil-water separation ofthe effluent from the anode had a capacity of 36 L and the gas-liquidseparator 33 had a capacity of 10 L.

The wastewater containing an emulsified oil which was to be treated inExample 6 was warm cleaning water containing a water-soluble cutting oilof type W1. This cleaning water was subjected to continuous oil-waterseparation in the following manner. First, city water was charged intothe cleaning tank 1 and the oil-water separation tank 2 for the effluentfrom the anode compartment. The total water charge was 240 L. The waterwas held at 50° C. by means of the electric heater build in the cleaningtank 1. In the next step, a water-soluble cutting oil of type W1, No. 1available from company X was injected continuously into the cleaningtank 1 at a flow rate of 3.0 mL/min. The water in the cleaning tank 1was stirred with an agitator to disperse the water-soluble cutting oil.

The cleaning water thus prepared was forced by the pump 9 to flow at arate of 16 L/min while electrolysis was conducted at a constant currentof 50 amperes, with the valve 34 being adjusted to provide a flowquantity of 8 L/min from the gas-liquid separator 33 to the packedcolumn 25 whereas the valve 35 was adjusted to provide a return flow of8 L/min to the suction port of the feed pump 9. At the start of theelectrolysis, the electrode 14 was the anode whereas the electrode 16was the cathode, with valves 26, 28 and 31 being fully open but valves27, 29 and 30 fully closed. The effluent from the cathode compartmentwas returned to the cleaning tank 1 at a flow rate of 0.9 L/min with thepH being in the neighborhood of 11.0.

When the electrolysis started, the pH of the effluent from the anodecompartment declined gradually and it was 5.5 after the lapse of 1 h.Then, valve 12 was operated to have the effluent return continuously tothe cleaning tank 1 such that its flow rate would be 0.9 L/min.Thereafter, the pH of the effluent stabilized in the neighborhood of6.0.

Every 12 hours of the electrolytic treatment, the polarities of the twoelectrodes were altered. On the first occasion, the output polarity ofthe dc power supply was changed and, at the same time, the electrode 14was switched to cathode whereas the electrode 16 was switched to anode,with the valves 26, 28 and 31 being rendered fully closed and the valves27, 29 and 30 fully open. Subsequent polarity changes were made byreversing the procedure of the previous operation.

At intervals of several hours in the continuous test, the oil separatedby flotation in the separation tank 2 was discharged and analysis of theoil concentration of the cleaning water in the tank 1 was carried out.

FIG. 6 shows the time-dependent profile of the oil concentration of thecleaning water.

Comparative Example 6

In order to further clarify the effectiveness of the fourth aspect ofthe invention, a comparative test for oil-water separation was made withthe same apparatus and under the same conditions as in Example 6, exceptthat no gas-liquid separator was provided. The data of the comparativetest are also shown in FIG. 6.

According to the fourth aspect of the invention, the followingadvantages are obtained.

a) The flow rate of the liquid on the anode side of the diaphragm in theelectrolyzer can be increased without increasing the quantity of theflow to the packed column or the tank for oil-water separation of theeffluent from the anode compartment and, hence, the chance of membranecontamination becomes even smaller than in the third aspect.

b) There is provided an oil floating action in the gas-liquid separatordue to the tiny oxygen bubbles in the wastewater which have beengenerated by electrolysis.

c) The throughput of the overall system can be increased by merelyincreasing the sizes of the diaphragm electrolyzer and the feed pump.

What is claimed is:
 1. A method for oil-water separation of a cleaningsolution in an oil-water separation step as it comes from a workcleaning step together with an oily contaminant, said method comprisingfeeding the contaminated cleaning solution into the anode compartment ofa diaphragm electrolyzer that is supplied with a current due to theapplication of a dc voltage between an anode and a cathode, heating thecleaning solution after it has passed through the anode compartment,introducing the heated cleaning solution to the oil-water separationstep, where oil is separated from the water, feeding the oil-freecleaning solution into the cathode compartment of the diaphragmelectrolyzer for neutralizing, and returning the neutralized cleaningsolution to the work cleaning step.
 2. A method according to claim 1,wherein said cleaning solution is water.
 3. A method according to claim1, wherein said cleaning solution is water having a surfactantincorporated therein.
 4. A method according to claim 1, wherein saidcleaning solution is water having a nonionic surfactant incorporatedtherein.
 5. A method according to claim 1, wherein said cleaningsolution is water having incorporated therein both a nonionic surfactanthaving a cloud point of 20°-40° C. and a nonionic surfactant having acloud point of 40°-80° C.
 6. A method according to claim 1, wherein saidcleaning solution is water having incorporated therein a nonionicsurfactant having a cloud point of 20°-40° C., a nonionic surfactanthaving a cloud point of 40°-80° C., and a builder.
 7. A method accordingto claim 6, wherein said builder is sodium sulfate.
 8. An apparatus foroil-water separation of a cleaning solution, which comprises cleaningsolution tank, a diaphragm electrolyzer having an anode and a cathodeprovided in an anode compartment and a cathode compartment,respectively, that are spaced apart by a porous diaphragm, and anoil-water separation tank, said apparatus being so adapted that acontaminated cleaning solution is fed from said cleaning solution tankinto the anode compartment of said diaphragm electrolyzer, from which itis fed to the oil-water separation tank through an inlet and emergestherefrom through an outlet to be fed into the cathode compartment ofsaid diaphragm electrolyzer for neutralization, with the neutralizedcleaning solution being returned to said cleaning solution tank.